1. Field of the Invention
The present invention relates to corrosion prevention, or more particularly to a method for preventing the corrosion of iron-containing vessels within an ethylene glycol distillation apparatus. This method includes the addition of an additive component to the apparatus, which additive component reacts with iron-containing inside walls of the apparatus vessels. This reaction forms a protective coating on these inside walls, to thereby protect them against corrosion. The additive component also prevents the decomposition of glycols, caused by the presence of iron in an ethylene glycol containing fluid flowing within the apparatus.
2. Description of the Related Art
A variety of ethylene glycol processes are known in the art. Typically, ethylene glycol is produced in a reactor, by the hydrolysis of ethylene oxide in a water stream. The result is an aqueous glycol-containing stream. This process often also result in the formation of light acids, such as acetic acid and formic acid, in the ethylene oxide reactor. It is economically desirable to produce ethylene glycol without the need for first purifying the ethylene oxide to remove all of its impurities. Thus, various distillation apparatuses for distilling ethylene glycol have been developed. For example, the aqueous glycol-containing stream may be fed into a series of multi-effect evaporator vessels to remove water, resulting in a concentrated glycol stream. This concentrated glycol stream may then be distilled by a series of reboilers and separated into monoethylene glycol, diethylene glycol, and triethylene glycol end products.
Additional output streams from an ethylene oxide reactor, which may contain acid salts and glycol, can be treated in an ion exchange unit to remove the acid salts and recover glycol. The treated output stream, although free of acid salts, contain esters of acids. While some of the esters are hydrolyzed and removed in the ion exchange unit, the bulk of them pass through this unit. The treated output stream, along with a distillation recycle stream, is fed into the multi-effect evaporator vessels to remove water from the glycol. The distillation recycle stream also contains glycolic esters of heavier acids such as glycolic acid and oxalic acid. The glycolic esters are hydrolyzed at high temperatures in the evaporator vessels, releasing free acids. Unfortunately these acids corrode the carbon steel of the evaporator vessels. In addition, undesirable iron products, generally in the form of magnetite, are carried with the concentrated glycol stream and coat the inside walls of downstream reboilers. At high reboiler temperatures the catalytic effect of the iron products causes the decomposition of glycol, forming carbon and aldehydes. Resulting carbon deposits on the reboiler walls necessitate frequent cleaning, which shortens the on-stream time of the reboilers. Furthermore, aldehydes generated by the glycol decomposition enter the monoethylene glycol (MEG) product, affecting the product quality. In fact, some plants require repeated cleaning of the reboilers just to maintain the product specification.
It would therefore be desirable to develop a method for preventing corrosion of iron-containing apparatus components, and for preventing glycol decomposition caused by catalytic interaction with iron. The present invention provides a solution to these problems.
It has now been found that the addition of a composition of sodium nitrite and sodium hypophosphite into the distillation apparatus reduces the corrosion rate in iron-containing vessels and reduces the catalytic effects of iron corrosion products in the downstream reboilers. This not only extends the on-stream time of the reboilers, but also improves the product quality by reducing the aldehyde content of the MEG product.